Electric vehicles and hybrid vehicles, which are equipped with an AC motor as their main engines, have received a lot of world attention in view of social requirements of low-fuel consumption and low emission. For example, a hybrid vehicle is composed of a DC power source, such as a secondary battery, an AC motor, and a control apparatus including a power converter, such as an inverter, and a controller. The inverter is connected between the DC power source and the AC motor. The controller controls the inverter to convert a DC voltage supplied from the DC power source into an AC voltage, and supplies the AC voltage to the AC motor, thus driving the AC motor.
The control apparatus for controlling an AC motor installed in a hybrid vehicle or an electric vehicle usually carries out feedback control of at least one controlled variable, such as a rotational speed or torque of the AC motor, using each phase current measured by a current sensor.
For example, when the control apparatus drives the inverter in a sinewave PWM control mode, the control apparatus converts each phase current measured by the current sensor into a d-axis current value and a q-axis current value in a d-q coordinate system, i.e. a two-phase rotating coordinate system, defined relative to a rotor of the AC motor. The control apparatus feeds back the d-axis current value and the q-axis current value from the AC motor to compare the d-axis current value and the q-axis current value with corresponding d-axis command current value and q-axis command current value determined based on a command value for the at least one variable of the AC motor. The control apparatus controls pulses, i.e. PWM pulses, to be supplied to switching elements of the inverter based on results of the comparison to control duty, i.e. duty factors, of the switching elements, thus converting a DC voltage input to the inverter into a sinewave AC voltage as an output of the inverter.
As another example, when the control apparatus drives the inverter in a rectangular control mode, i.e. a single-pulse control mode, the control apparatus estimates, based on a d-axis current value and a q-axis current value fed back from the AC motor, torque of the AC motor as the at least one variable. The control apparatus feeds back the estimated torque from the AC motor, and compares the estimated torque with request torque. Thus, the control apparatus controls operations of the switching elements of the inverter based on results of the comparison to thereby adjust a value of the estimated torque to a value of the request torque.
On the other hand, there is a motor control technology using the Fourier transform. For example, Japanese Patent Application Publication No. 2010-57218 discloses a technology that applies Fourier transform to a value of torque of an AC motor directly measured by a torque meter or a value of torque estimated based on a measured current value, thus extracting a desired-order signal component. Then, the technology performs learning for a power converter such that the coefficient of the desired-order signal component becomes zero in order to reduce torque ripples.